Conventionally known is a three-times transfer type color electrophotographic method in which, using powder toners with three colors, i.e., yellow (Y), Magenta (M) and cyan (C), overlaps toner images with three colors on a transfer sheet by, for each toner, repeating three times an electrophotographic process including charge, color-separation exposure, development, transfer and cleaning.
In this system, it should be required to transfer on a transfer sheet each of three-color toner images to be successively formed on the photoconductor without occurrence of position slippage. This provides a problem that a transfer drum in addition to the photoconductor is required to cause the apparatus to increase in size and become complex.
Accordingly, in order to remove the aforementioned problem are proposed various one-time transfer type color electrophotographic methods which do not require the transfer drum. That is, this is a system in which a toner-image forming cycle including electrification, exposure and development is repeated several times so as to form on the photoconductor a plurality of toner images which are different in color from each other before collectively transferring them to a transfer sheet. One well known example of such systems is disclosed in U.S. Pat. No. 4,599,286.
For such a one-time transfer system, one important point for determining the color image quality is to perform electrification, exposure and development on the photoconductor having a toner image so as to faithfully form a toner image on the first-mentioned toner image with respect to an optical image, the toner image to be formed being different in color from the first-mentioned toner image. Therefore, the optical characteristic of a toner layer making up the toner image results in being extremely important.
However, since performance required for the toner layer is not known conventionally, it is difficult to obtain a clear full-color image. Particularly, there are problems that the color tone of a mixed color image formed by overlapping of a color-different toner image varies and nonuniformity occurs in color, thereby causing extreme deterioration of the image quality of a full-color image.
A description will be given hereinbelow in terms of the problems. In the development process of the conventional one-time transfer system, as disclosed in the above-mentioned U.S. Pat. No. 4,599,286, the two-component magnetic brush developing method is general where a two-component developer, being a mixture of a carrier magnetic powder and a toner, is used and the development is effected with the developer being spiked by a magneto. A description will be made with reference to this two-component magnetic brush developing method in terms of the case of forming a mixed color image in which a first toner image is overlapped with a second toner image whose toner layer thickness is constant.
When a photoconductor holding the first toner image is charged and then the first toner image is exposed from the upper side with a second optical image with constant quantity of light and developed with a second toner, the adhesion amount of the second toner varies in accordance with the toner layer thickness of the first toner image. That is, in order to obtain a desirable mixed color image, it is essentially required that the adhesion amount of the second toner is constant irrespective of the toner layer thickness of the first toner image. However, the second toner is adhered to be relatively thick at a portion that the toner layer thickness of the first toner image is small. On the contrary, the second toner is adhered to be relatively thin at a portion that the toner layer thickness of the first toner image is great.
According to analysis of this cause, it has been found that the adhesion amount of the second toner is decreased exponentially in accordance with increase in the toner layer thickness of the first toner image, more specifically the number of the laminated toner layers. This is due to the fact that the scattering quantity of light passing through the toner layer increases as the number of toner layers of the first toner image and hence the quantity of light passing through the toner layer and reaching the photoconductor exponentially decreases to cause the surface potential of the photoconductor to vary. For example, in the case of a yellow toner with an average particle diameter of 10 .mu.m, when the quantity of light transmitting one layer (toner covering rate: 50%) is taken as 100%, two layers result in about 20% and three layers result in several %. Furthermore, when the photoconductor is charged by means of a corona charger in order to form a second toner image, the charged amounts of the first toner and the photoconductor are distributed in inverse proportion to the electrostatic capacities of the first toner layer and the photosensitive layer. Therefore, in cases where the toner layer thickness of the first toner image is nonuniform, it is understood that the charged amount of the photoconductor itself becomes nonuniform.
Accordingly, in order to attain a clear mixed color image with the one-time transfer system, it is required to use a toner which can provide a high density irrespective of a small number of layers and further employ a developing method which is capable of performing the development so that the toner layer thickness is small and uniform.
However, conventional color toners are arranged so that the maximum density can be first obtained with overlapping of four to six layers. In addition, in the two-component magnetic brush developing method, the adhesion amount of a toner to be developed varies delicately due to variation of the mixing ratio of the toner and carrier and others even if the surface potential of the photoconductor is constant, and therefore, it is extremely difficult to develop the toner layer to be thin and uniform. Moreover, with respect to a high density image in which the toner adhesion amount becomes great, the toner layer thickness varies so as not to become constant.
From the above-described reasons, when reproducing a high-density mixed color image with the conventional one-time transfer system, the one-color toner layer thickness becomes great and the toner layer thickness greatly varies, and therefore, the color tone may vary and color nonuniformity occurs.